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Controlling Alternaria leaf blotch and fruit spot without fungicides

Biosecurity

Gordon Brown reviews research investigating how Alternaria infects apple leaves and fruit.

Researchers in Italy have identified that we do not have a sound knowledge of the infection process of apple leaves and fruit with Alternaria. They considered this important as Alternaria is normally a saprophytic fungi and considered a weak pathogen. One of the features that puzzled them is that there is a poorly understood physiological disorder, ‘leaf blotch’, which has identical symptoms and climatic conditions as Alternaria leaf blotch. Physiological leaf blotch is thought to be due to nutrient imbalances and it is known that zinc- or manganese-containing fungicides as well as magnesium and manganese foliar fertilisers reduce the incidence of physiological leaf blotch, suggesting these nutrients are involved in the development of the disorder.   

As a result the researchers developed a hypothesis that physiological and Alternaria leaf blotch are two interlinked diseases with initial leaf blotch symptoms being due to nutrient imbalances and these are subsequently colonised by Alternaria to become Alternaria leaf blotch. 

To start this study the researchers collected leaves of Golden Delicious from orchards at the Laimburg Research Centre in South Tyrol with either low incidence of leaf blotch or high incidence of leaf blotch and analysed them for major and minor elements. This revealed that orchards with a high incidence of leaf blotch had low levels of foliar sulphur, magnesium and manganese (figure 1). Other nutrients were similar between healthy and affected orchards.

 

Figure 1: Impact of foliar levels of sulphur, magnesium and manganese on foliar area with symptoms of leaf blotch.

 

The next thing the researchers did was to isolate Alternaria from the leaf blotches and fruit spots, and identify them down to subspecies level. In all, four different Alternaria isolates were identified. They then inoculated healthy leaf discs in Petri dishes with the four different isolates of Alternaria and, despite thick mycelium growing on the leaf surface, no leaf necrosis developed, indicating the fungi could not infect the intact leaves.  

 

Alternaria fruit spot

 

With these leaf infection studies it was observed that leaves with symptoms of European red mite damage did develop Alternaria leaf blotch in this test, supporting the hypothesis that the leaf needed to be damaged for Alternaria to infect it. To study this in more detail leaf spots were induced on trees in July using a herbicide. In September leaves with Alternaria blotch symptoms were collected from these trees and it was found that the spore density on herbicide-induced necrotic spots was 14 times greater than on naturally occurring blotched leaves (figure 2). These results demonstrate that pre-existing necrosis on the leaves leads to subsequent Alternaria infection.

 

Figure 2: Impact of herbicide-induced necrotic spots on density of naturally occurring Alternaria spores 2 months after foliar damage induction.

 

Finally the researchers conducted field trials where they applied sulphur-based foliar nutrient sprays to the trees and later in the season they recorded the level of Alternaria leaf blotch and fruit spot symptoms. The sulphur mix consisted of 3800g magnesium sulphate, 33g manganese sulphate and 115ml zinc sulphate in 100 litres of water. Hence this mix supplied magnesium, manganese and zinc as well as sulphur to the trees. To verify that this mix was not fungicidal towards Alternaria it was added to PDA fungal nutrient in a Petri dish and the growth of Alternaria studied. The sulphur mix did not inhibit Alternaria and if anything improved its growth in the Petri dish. This confirmed that the foliar sulphur fertiliser used did not have any direct fungicidal impact. Any effect noted on trees would be due to physiological changes in the trees due to improved tree nutrition. Sprays were applied at 1500L/ha starting from early summer every three weeks till harvest and was repeated for three years. In the first season five sprays were applied and in the remaining two seasons four sprays were applied.   

Simultaneously in the same orchards in the same years a separate trial was conducted investigating the impact of the fungicides potassium phosphonate and Difenoconazol on the appearance of leaf blotch. These materials were applied after rain and 8, 5 and 11 applications were made in each of the three seasons. It is not clear when the first fungicide spray was applied. 

These trials found that the fertiliser treatment significantly reduced the appearance of both leaf blotch and fruit spot. The foliar disease control achieved with the 4–5 sprays of fertiliser was almost double that of the 8–11 fungicide sprays indicating that a reduction in pesticide applications is possible by the use of foliar fertiliser. Unfortunately the level of disease control on the fruit achieved with the fungicide treatments was superior to the fertiliser treatment (figure 3). The researchers explain their thoughts for the reduced control of fruit spot with the fertiliser treatments compared to control on the leaves as being due to the fact that Alternaria infects fruit through lenticels which are not impacted by the fertiliser. It is thought that the reduction in fruit spot observed was through reduced spore load from the leaves rather than a direct control of the disease on the fruit.

 

Figure 3: The impact of spraying foliar fertiliser based on S, Mg, Mn and Zn (up to 5 applications) or fungicides (up to 11 applications) on the appearance of leaf blotch and fruit spot symptoms on Golden Delicious trees.

 

Conclusion

These results provide compelling evidence that Alternaria leaf blotch is due to Alternaria infecting already damaged leaves and not due to a direct infection of healthy leaves. Hence treatments that reduce foliar damage or improve tree nutrition, such as ensuring adequate sulphur, magnesium, manganese or zinc nutrition, will dramatically reduce the appearance of Alternaria leaf blotch and subsequent Alternaria fruit spot. 

 

Alternaria leaf blotch.

 

Take home messages

  • Alternaria does not infect healthy leaves. 
  • Alternaria infects leaves through existing necrotic spots. 
  • Ensuring adequate tree nutrition, especially with regards to S, Mg, Mn and Zn, will reduce the appearance of Alternaria leaf blotch. 
  • Minimising other stresses, such as insects or hail damage, that lead to necrotic spots will reduce the appearance of Alternaria leaf blotch. 
  • Reducing the incidence of Alternaria leaf blotch will reduce the appearance of Alternaria fruit spot. 


Further reading
 

Source information: 

Additional information: 

 

This article was first published in the Summer 2023/4 edition of AFG.

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